The normal intervertebral disc is a highly specialized joint between the intervertebral bodies from the second cervical vertebra to the first sacral vertebra in the human body. A disc is made up of a strong outer ring called the annulus, which is strongly attached to the intervertebral bodies above and below through collagen fibers, and a central nucleus. The nucleus comprises a mesh of collagen fibers to which is attached proteoglycan molecules which are hygroscopic. It can therefore be said to consist of a central portion and an intermediate zone. The central portion comprises 90% proteoglycan and 10% collagen, and the intermediate zone rather less proteoglycan and proportionately more collagen. The annulus comprises 90% collagen with about 10% proteoglycan, which proteoglycan acts to allow a sliding motion between adjacent layers during normal daily use.
Like all other joints in the body, intervertebral discs, particularly lumbar intervertebral discs, are subject to various types of injury, degeneration and disease. Painful disc syndromes can develop due to the destruction of the intervertebral disc structure.
It is often the case that back pain emanating from an intervertebral disc arises from a damaged annulus because the outermost third of the annulus itself is the only part of the disc structure which is innervated. In theory then, the insertion of a prosthesis which has itself a finite structure with inherent strength but at the same time is held in position by the annulus would permit the annulus itself to heal with the commensurate relief of pain.
Such a prosthesis has been suggested by the present applicant in WO 99/02108. This provides an intervertebral disc nucleus prosthesis generally comprising a transudative or ion transport material extended over the inner face of a prosthetic woven or porous fabric chosen for its strength and immunological neutrality. With this design, tissue can grow over and incorporate the outer fabric, while water and very low molecular weight materials can pass through the prosthetic fabric and the transudative membrane, thus eliminating, or at least substantially alleviating, any immunological problems. This allows the anchoring of the replacement nucleus pulposis within a disc space while preventing the ingrowth of bony trabeculae, thereby providing a more satisfactory long term solution.
An alternative arrangement generally comprises a three layer cover, wherein the outer and inner layer are formed of a strong material in a sandwich construction with a middle layer of a transudative material of a small pore size. Very high pressures are present in use within the normal disc nucleus. To counteract this it is often necessary to introduce a hydrogel or other suitable substance into the cover in a liquid or powdered state. Thus, the hydrogel would have a tendency to escape through the cover were it not so constructed as to combine strength with an ability to retain the hydrogel which might otherwise leak out through the pores in the cover. This alternative jacket construction addresses this concern. Furthermore, materials have been developed which have both strength and also function as a transducive membrane, e.g. Goretex.
Another alternative arrangement is to have a cover of a relatively impermeable material, for example polyurethane, which is both strong and bio-compatible. In such arrangements, a hydrogel and water mixture can be added in a pre-defined amount and ratio, so as to produce a consistency which substantially replicates the properties of the natural nucleus.
The various prosthetic arrangements described above remain in position due to the snug fit within the annulus, and by virtue of the prosthesis being considerably larger than the hole in the annulus through which it was originally inserted. Also, adhesions will develop between the prosthesis and the annulus over time.
Orientation of the prosthesis is facilitated by introducing apparatus including a tubular external valve body which may be internally screw threaded. An introducer rod is passed through the body and may have an external screw thread for engagement therewith. The rod thus serves as a stiffening device for positioning the cover in the disc cavity. Where screw threads are provided, rotation of the rod extends or retracts the distal end of the rod further to facilitate orientation of the cover. The cover preferably has a strengthened portion opposite the valve in which the distal end of the rod may be seated.